Preheat grill or broiler. Lightly score edges of steak at 1 inch intervals, to prevent curling. Combine butter, parsley, onion, worcestershire sauce, pepper and mustard in a small saucepan. Heat, stirring continually, over low heat, until butter melts. Reserve 1/4 of the mixture. Place steak on grill or broiler pan. Brush steak with the butter mixture. Cook, basting frequently with the butter mixture, about 6 to 8 minutes per side depending how you like your steaks done. Place steak on a serving platter. Cut thin slices across the grain of the steak. Drizzle reseved butter mixture over steak.